1. Field of the Invention
The invention relates to the field of glazing, especially automotive glazing, with very little double imaging.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Automotive glazing, especially of the windshield or rear window type, must have the smallest number of optical defects for both safety reasons and esthetic reasons. The vision of a driver must be as clear as possible and it is in particular unacceptable for the image that he perceives of the environment of the automobile to be a double image. Multiple imaging as perceived by the driver through the windshield is a known phenomenon stemming from multiple reflections at the air/glass interfaces. It is generally referred to as double imaging even though in theory other additional images exist, since these additional images are of very low intensity. An additional reflection is accompanied in fact by a substantial loss of intensity of the parasitic image, by a factor of the order of 100 relative to the intensity of the main image. The magnitude of the phenomenon increases with the angle of inclination of the glazing.
The theoretical amount of double imaging, which is expressed as a number in minutes by those skilled in the art, can be calculated by computer. This amount depends on many factors, such as the thickness of the panes constituting the windshield and on the local curvature, but also on the angle of viewing through the glazing. A distinction is made between the amount of vertical double imaging (the images appearing one above the other when a person is sitting in the vehicle) and the amount of horizontal double imaging (the images appear one beside the other when a person is sitting in the vehicle). The usual inclination of the windshield generates vertical double imaging, whereas in general there is no horizontal double imaging problem. The closer the glazing is to the horizontal on the motor vehicle (as for example in FIG. 3), the greater the amount of vertical double imaging. At the present time, the maximum acceptable amount of double imaging (in all directions) is considered to be 7 minutes, as this is deemed to be imperceptible by the human eye. It is the amount of vertical double imaging that is difficult to contain to within at most 7 minutes in the case of glazing which is highly inclined in the use position. The amount of double imaging may be measured using the target test technique or the collimation telescope test technique as described in Rule 43, Addition 42 of the E/ECE/324 or E/ECE/TRANS/505 agreement, relating to the adoption of uniform technical requirements applicable to wheeled vehicles, to equipment and to components that can be mounted or used on a wheeled vehicle, and the conditions for reciprocal recognition of homologations granted in accordance with these provisions.
Automobile manufacturers, especially French ones, seek to design ever more innovative models. In particular, the windshields designed may be very large, since sometimes they may even form part of the roof by going over the top of front passengers. These windshields are also increasingly inclined to the horizontal. Moreover, their curvatures must be very regular so as to merge into the general shape of the automobile.
In the context of developing such windshields, it has been discovered that the amount of vertical double imaging is greater than the theoretical amount of double imaging without knowing the reason why. The windshields in question have been produced by a gravity bending process, which theoretically should be very suitable for forming them. The amount of vertical double imaging was more than 50% greater than the theoretical amount of double imaging.
Many bending processes have already been described, such as gravity bending as in EP 0 448 447, EP 0 705 798 and WO 2004/103922, bending by running glass between conveying rollers as in WO 2004/033381 or WO 2005/047198, and bending by pressing glass against a solid mold, said pressing being carried out either using a frame, as in U.S. Pat. No. 5,974,836, WO 95/01938, WO 02/06170 or WO 2004/087590, or by suction as in WO 02/064519 or WO 2006/072721.